risk_free_rate as a series

[xavier676]

Hello all 👋

First, thank you for this amazing library!

I’d like to suggest a possible improvement: currently, the risk_free_rate is treated as a static value, but it could be useful to allow it to be passed as a time series instead. Over long periods, the risk-free rate can vary significantly, and supporting a series would make results more realistic.

What do you think?

Best,